A molecular theory of smectic C liquid crystals made of rod-like molecules

Abstract

Organic compounds exhibiting the smectic C phase are made of rod-like molecules that have dipolar groups with lateral components. We argue that the off-axis character of the lateral dipolar groups can account for tilt in layered smectics (SmC, SmC*, SmI etc.). We develop a mean-field theory of the smectic C phase based on a single-particle potential of the form UC is proportional to sin(2theta) cos phi, consistent with the biaxial nature of the phase, where theta and phi are the polar and azimuthal angles, respectively. The hard-rod interactions that favour the smectic A phase with zero tilt angle are also included. The theoretical phase diagrams compare favourably with experimental trends. Our theory also leads to the following results: i) a first-order smectic C to smectic A transition above some value of the McMillan parameter alpha, leading to a tricritical point on the smectic C to smectic A transition line and ii) a first-order smectic C to smectic C transition over a very small range of values of the model parameters. We have also extended the theory to include the next higher-order term in the tilting potential and to include the effect of different tilt angles for the molecular core and the chain in the SmC phase.

Two-dimensionally periodic phases in mixtures of compounds made of rodlike and bent-core molecules

“…The aromatic cores are associated with several dipolar groups having components in the molecular plane and these can give rise to a tilting about the arrow axes to reduce electrostatic repulsion energy. This dipolar mechanism is similar to the case of the smectic-C liquid crystal formed by R molecules, where the off-axis components of the dipoles can produce tilting of the molecules [18]. Another contributory factor could be a larger molecular cross-section of chains compared to the cores, which can also produce tilt [19].…”

Tilted and Non-tilted Liquid Crystalline Langmuir Monolayers: Analogy to Bulk Smectic Phases

“…2) Since the late twenties, both the experimental [3][4][5] and theoretical 1,6,7) works have been actively done on the phase transition especially from smectic-A to smectic-C phases, which induces the abrupt or continuous molecular tilt from zero to a certain angle from the layer normal. The phenomenological models based on the continuum theory could explain and reproduce the experimental results satisfactorily, 1,6,7) while several molecular models [8][9][10] were also proposed to explain why the rod-like molecules should spontaneously tilt in smectic-C phase. In the successful molecular models, [8][9][10] always the common assumption is applied tacitly; the molecular tilt in smectic phases is not influenced by the interlayer interaction but is governed only by the intralayer molecular correlation.…”

“…The phenomenological models based on the continuum theory could explain and reproduce the experimental results satisfactorily, 1,6,7) while several molecular models [8][9][10] were also proposed to explain why the rod-like molecules should spontaneously tilt in smectic-C phase. In the successful molecular models, [8][9][10] always the common assumption is applied tacitly; the molecular tilt in smectic phases is not influenced by the interlayer interaction but is governed only by the intralayer molecular correlation. This assumption has been used with no certain proof because it sounds reasonable and also the confirmation by using the single smectic layer is not easy.…”

Evidence of antiferroelectric-like ordering from the study of dielectric permittivity–voltage behavior in bent core liquid crystalline material showing B2phase

“…The aromatic cores are associated with several dipolar groups having components in the molecular plane which can give rise to a tilting about the arrow axis. This dipolar mechanism is similar to the case of the smectic C liquid crystal formed by rod-shaped molecules, where the off-axis components of the dipoles can produce tilting of the molecules [18,19]. On the other hand, larger molecular cross-section of chains compared to the cores in these types of molecules can also produce tilt [18].…”